On 6/3/2019 9:21 AM, Hare, Ed W1RFI wrote:
There are a number of possibilities here.
I got involved in trying to help a Solar Edge applications engineer
optimize a couple of choke designs to be applied to the loop of
optimizer outputs on the roof. He provided me with lengths of the cable
they are required to use (by code). In the process, we had discussions
at a couple of long lunches (he lives in Oakland, CA, about 70 miles N
of me), and I made several major points to him.
First, that the most cost effective way to solve these problems is to
design the electronics to minimize MF and HF content, and to practice
good EMC practices in mechanical/electrical product design. This would
include interior layout of everything in the path of potentially noisy
current, careful shaping of the switching waveform to minimize harmonic
content (as first Elecraft, and subsequently Flex) have done with CW
keying), and, of course, suitable filtering.
Second, he described a bonding architecture for the panels in the
particular installation in question whereby lightning ground and power
ground were on opposite sides of the building, and said that had been
dictated by the inspector. This arrangement could be producing both
magnetic and EM fields. As part of troubleshooting, I suggested that the
lightning ground temporarily be moved so that it follows the same path
as the switched DC conductors.
Noise generated by the panels themselves can, of course, be radiated by
conductors within the panels, and by wiring external to the panels. In
addition, the system can, if there is a sufficiently large loop, produce
a magnetic field which can be quite strong in the near field. There are
also electrical/mechanical designs that can minimize both the magnetic
and EM fields.
An example of what I'm getting at is careful implementation of
microstrip techniques, either on circuit boards, or by routing
conductors in very close proximity to a continuous conducting surface
(like a metallic enclosure or frame). This causes return current to flow
in the adjacent conducting surface, forming a transmission line, which
in turn minimizes both the magnetic and EM fields.
I've not heard from that systems engineer since my choke design was
applied to the installation in question, but I've heard separately
(perhaps here?) that they were redesigning some of their electronics.
And, of course, there are multiple potential sources of EMI -- the
panels, the optimizers, the inverter, and any control circuitry.
A common-mode choke is only going to be effective if it includes hot, neutral
and ground for ac wiring and +, - and ground for dc.
Exactly right. But for purposes of clarity of understanding, I prefer to
use the word "return" in place of "ground" when talking about DC wiring.
:) For the same reason, I don't like the word "balun" -- I can think of
nearly a dozen VERY different things commonly described by that word, so
using obscures understanding of what they are and how they work. :)
Another point that I emphasized to that applications engineer was that
it is NOT, DC wiring, it is, by virtue of the continuous switching of
the waveform, both DC and RF wiring.
73, Jim K9YC
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